Plastids are major organelles found in plants including algae. The term plastid was coined by E. Haeckel in 1866.
These are double membrane organelles with their own DNA and are responsible for photosynthesis, storage of starch and for synthesis of many classes of molecules such as fatty acids and terpenes. Some protists like Euglcna, Dinophyccac and diatoms also have plastids.
Plastids are the largest cell organelles. All plastids are derived from proplastids, formerly known as eoplasts (eo-early). In plants plastids may differentiate into several forms depending upon the types of function they need to play in the cell. Undifferentiated proplastids may develop into any of the following forms:
Chloroplasts: for photosynthesis. The chloroplast with nitrogen fixing genes (nif genes) constitutes nitroplast.
Chromoplasts: for pigment synthesis and storage. The pigments associated with them are xanthophylls (yellow) and carotene (orange – red) as seen in colored petals and fruits.
Lcucoplasts: for monoterpene synthesis. Leucoplasts sometimes differentiate into more specific plastids like:
♦ Amyloplasts: for starch storage as in the case of potato tubers, wheat and rice grains.
♦ Elaioplasts: for storing fats as in the case of seeds of castor and peanuts.
♦ Proteinoplasts: for storing and modifying proteins. The aleuronoplasts of maize is one such example.
Plastids have the ability to differentiate and re dedifferentiate to different forms. The chromoplasts may be developed either form leucoplasts or chloroplasts. The lamellae degenerate partially or completely during chromoplast development. When green fruits ripen they become variously colored due to transformation of green chloroplast to the colored chromoplasts.
Further in algae the term leucoplast (=leucoplast) is used for all unpigmented plastids. Their function differs from the leucoplasts in plants. Etioplast, amyloplast and chromoplast are found in higher plants but do not occur in algae.
The chloroplast in algae other than green algae is called chromatophores (e.g. rhodoplast in red algae and phoeoplasts in brown algae). The plastids of algae may also differ from those of the plants in that they contain pyrenoids (Beyond algae, Pyrenoids are also found in the chloroplasts of Anthoceros (Bryophyte) and Selaginella (Pteridophyte).
Inheritance of Plastids
Mostly plastids are inherited through maternal inheritance or cytoplasm inheritance. Many gymnosperms, however, inherit plastids from male parent. In algae also the plastid inheritance is from one parent only. The plastid DNA of the other parent is thus completely lost.
Origin of plastids
Plastids are thought to have originated as endo-symbionts. Presence of plastid DXA and their division independent of cell division cycle are some of the evidences of their endo-symbiotic origin. Some dinofiagellates take up algae as food but retain the plastids of the digested algae to profit from photosynthesis, (after a while the plastids are also digested).
Apicomplexa, the obligate parasitic protozoan which include the causative agents of malaria and many other human or animal diseases also harbor a complex plastid called apicoplast. The apicoplast is not capable of photosynthesis and is a promising target for antiparasitic drug development. Such type of plastid also point to the endo-symbiotic origin.
Chloroplasts: Schimper (1883) coined the term chloroplast. These are the photosynthetic pigment containing plastids found in green algae and plants. The shape, size and number of the chloroplast vary greatly.
In higher plants the chloroplasts are mainly ovoid, spherical, discoid or lens-shaped. In algae, the shape shows a great variation. It is cup-shaped in Chlorella and Chlamydomonas, girdle-shaped in Ulothrix, star-shaped in Zygnema, spiral in Spirogyra, reticulate in Oedogonium and discoid in Vaucheria. Generally the chloroplast is about 4-8m in size but in polyploids the size is bigger.
In sciophytes (shade plants) chloroplasts are bigger than in heliophytes (light plants). A single chloroplast per cell is found in some algae like Chlamydomonas, Ulothrix, and Chlorella etc. Two chloroplasts per cell are found in Zygnema and 2-14 chloroplasts per cell are found in Spirogyra. In higher plants 20- 40 chloroplasts per cell have been reported.
Like Mitochondrion, the chloroplast is enclosed by two membranes. But the chloroplasts are larger and more complex than the mitochondria. In addition to the outer and inner membranes the chloroplasts also have a closed compartment of stacked membrane called grana (singular, granum) that lie internal to the inner membrane.
In one chloroplast there may be hundred or more grana. Each granum contains from a few to several dozen disc-shaped structures called thylakoids. The smaller thylakoids are placed one above the other like stacks of coins to form the granum.
The larger thylakoids arc large membranous structures known as inter granal lamellae or stroma lamellae or fret channels. Surrounding the thylakoid is a fluid matrix called stroma. The dark reaction of photosynthesis occurs here. Stroma is a proteinaceous matrix. The photosynthetic pigments are housed on the surface of thylakoids.
The space between the two membranes surrounding the chloroplast is known as periplastidial space. The algal chloroplasts are agranal as they lack grana.
Like the mitochondrion the chloroplast also contains its own DNA. Plastid DNA exists as large protein-DNA complex associated with the inner envelope and are called “plastid nucleoids’. Each nucleoid particle may contain more than ten copies of the plastid DNA. The proplastid contains a single nulcleoid located at the centre of the plastid.
The developing plastid has many nucleoids localized at the periphery of the plastid bound to the inner envelope. During differentiation of proplastids to chloroplasts as also when plastids convert from one type to another nucleoids change in morphology, size and location.
The chloroplast DNA contains genes for the synthesis of specific protein components necessary to accomplish photosynthetic reactions. Majority of the proteins needed in chloroplasts are synthesized by nuclear genes and imported into chloroplast.
There are approximately 120 genes in chloroplast DNA of which about 60 genes are involved in RNA transcription and translation including genes for rRNA, tRNA, RNA polymerase subunits and ribosomal proteins. About 20 genes encode subunits of chloroplast electron transport complexes and the ATPase complexes. The large subunits of the photosynthetic enzyme RUBISCO are also encoded by chloroplast genome.
Chemical composition: Chloroplast is constituted by 45-50% proteins, 20-25% phospholipids, 10% chlorophylls, 1-2% carotenes, RXA, DNA, enzymes, coenzymes, magnesium, copper, iron, zinc and manganese.